1. Field of the Invention
The present invention relates to a micro-mirror manufacturing method, and more particularly to a micro-mirror manufacturing method for aligning individual micro-mirror devices to a package substrate to mount them on it after dividing a plurality of micro-mirror devices formed on a wafer into them.
2. Description of the Related Art
Generally projectors using a spatial optical modulator, such as a transparent LC, a reflective LC, a micro-mirror array and the like are widely known.
The spatial optical modulator forms a bi-dimensional array on which several tens thousand to several millions of fine modulation devices are arrayed and each individual array is enlarged and displayed on a screen through a projection lens as each of pixels corresponding to an image to be displayed.
The spatial optical modulator used for a projector falls roughly into two of an LC device for modulating the polarization direction of incident light by enclosing/fixing an LC between transparent substrates and giving a potential difference between the transparent substrates and a micro-mirror device for controlling the reflection direction of illumination light by deflecting a fine micro electric mechanical systems (MEMS) mirror by electro-static power, which are generally used.
Patent Document 1 discloses one example of the micro-mirror device. In Patent Document 1, a drive circuit using a metal oxide semiconductor field-effect transistor (MOSFET) and a transformable metal mirror are formed on a semiconductor wafer substrate. This mirror can be transformed by the electro-static power of the drive circuit to change the reflection direction of incident light.
Patent Document 2 discloses an embodiment example for holding a mirror by one or two elastic hinges. When the mirror is held by one elastic hinge, the elastic hinge functions as a curved spring. When the mirror is held by two elastic hinges, the elastic hinges function as a twisted spring to deflect the reflection direction of incident light by tilting the mirror toward different directions.
The size of a mirror constituting the above-described micro-mirror device has each side of 4˜20 μm and the mirror is disposed on a semiconductor wafer substrate in such a way that a space in adjacent mirror surfaces can be miniaturized as much as possible. One micro-mirror device is made by forming an appropriate number of mirror elements including these mirrors as image display elements. In this case, the appropriate number as image display elements means, for example, a number based on the resolution of a display, which is stipulated by Video Electronics Standards Association (VESA) and a number based on the TV broadcast rating.
When constituting a micro-mirror device which has a number of mirror elements, corresponding to wide extended graphics array (WXGA) (resolution: 1280×768) stipulated by VESA and a mirror pitch of 10 μm, the diagonal length of its display area is approximately 0.6 inch. Thus sufficiently small micro-mirror device is made. Therefore, when actually manufacturing micro-mirror devices, from the viewpoint of productivity improvement, a plurality of micro-mirror devices are formed on one piece of a semiconductor wafer substrate at one time and are divided into individual micro-mirror devices.
The unit of division, that is, dicing is called “die”. When attention is paid to after dicing, an individual micro-mirror device separate from one piece of a semiconductor wafer substrate is sometime called “micro-mirror device die”.
Since an individual mirror in such a micro-mirror device is very tiny, the attachment of a little foreign object sometimes causes a poor operation. Especially, in the dicing process of dividing a semiconductor wafer substrate into individual micro-mirror devices, sometimes a mechanical defect caused by the dicing process enters an MEMS structure to cause a poor operation and sometimes destroys the MEMS structure itself. Various methods for preventing it are disclosed.
For example, Patent Document 3 discloses a technology for forming a first sacrificial layer and a second sacrificial layer on a semiconductor wafer forming the mirror element of a micro-mirror device by a photoresist process and removing the first and second sacrificial layers by cleaning it with hydrogen Fluoride (HF) after forming a scribe line. Patent Document 4 also discloses an embodiment example of forming a protection layer on a mirror in the mirror element formed on a semiconductor wafer by a photoresist process and removing photoresist when completing the electric connection to a package substrate after dicing it. Furthermore, Patent Document 5 discloses an embodiment example of forming an organic protection layer in which resin is mixed in a solvent on an MEMS device. Furthermore, Patent Document 6 discloses an embodiment example of forming a protection layer on a mirror in a mirror element formed a semiconductor wafer by vacuum evaporation.
Here, for example, a case as described in Patent Documents 3 and 4 where the reflection surface of a mirror in the mirror device of a micro-mirror device formed on a semiconductor wafer substrate is made of aluminum and photoresist is used as the protection layer of a mirror reflection surface is assumed and studied. In this case, as a method for removing the photoresist after dividing the semiconductor wafer substrate into individual micro-mirror devices there are two methods of a dry method and a wet method.
In the dry method, burning by oxygen plasma ashes is popular. However, in the dry method, there is a possibility of disturbing its optical usage since an aluminum mirror reflection surface distorts due to an inappropriate working condition and further undergoes oxidation by the reaction between the oxygen plasma and aluminum. Therefore, it is necessary to pay sufficient attention to the setting of the working condition.
In the wet method, there is a method for removing the photoresist using a solvent whose major component is a phenol and halogen family solvent in an organic family and a method for removing the photoresist using a mixed acid, such as a sulfuric acid hydrogen peroxide mixture (SPM), a hydrochloric acid hydrogen peroxide mixture (HPM), etc., an ammonia hydrogen peroxide mixture (APM) and the like in an inorganic family. Since the former organic halogen family solvent greatly affects an environment, recently it must be avoided to use it. Since the latter inorganic family mixed acid and the like corrodes the aluminum mirror reflection surface due to a sulfuric acid, hydrochloric acid and the like included in the mixed acid, there is a possibility of deteriorating the function of a mirror.
In an example of forming a protection layer in which resin is mixed in a solvent, which is disclosed in Patent Document 5, resin coating is applied again, including the space between the mirror and the substrate after temporarily releasing the mirror. Since in this process, there is a possibility that resin coating work itself may destroys the MEMS structure, sufficient attention must be paid.
Furthermore, according to Patent Document 7, when applying resin coating to this MEMS device, the resin protection layer deforms while dividing the semiconductor wafer substrate into individual MEMS devices and as a result, it does not function as the protector of the MEMS structure. Therefore Patent Document 7 further discloses a technology for coating a harder protection layer (photoresist) over on the resin protection layer in order to solve this inconvenience. However, it has a problem that work becomes complicated and troublesome.
The micro-mirror device die separate by the above-described method is attached to a package substrate and is further covered with a transparent substrate being a lid. Thus a micro-mirror can be disposed in an almost enclosed space. Thus, a package structure in which a micro-mirror stably operates without any influences of external force, dust and the like. In this case, the semiconductor substrate of a micro-mirror device die can be also used as a package substrate.
In order to improve the function as the whole micro-mirror device die, it is preferable not only to protect the micro-mirror device die from the influences of external power and dust by package it but also to correctly dispose it in the desired position of the package substrate. It is because it is preferable to dispose a mask for shutting unnecessary light and the micro-mirror device die in correct relative positions and to simplify aligning in the case of inserting the packaged device in a device, such as a projector and the like.
Therefore, it is preferable to position the micro-mirror device on the package substrate having high accuracy and fix it.
Patent Document 8 discloses an example used to position of the two sides of a chip (that is, die) for such alignment. Patent Document 9 discloses an example of adjusting their relative positions on the basis of an optical alignment mark.
However, in Patent Document 8 it is presumed that the relative positions between the side of a chip (that is, die) and its display surface should be accurately processed. For example, if a cheap process of putting a groove and dividing by an anvil when separating dies from the wafer is adopted, it cannot be expected to obtain necessary accuracy. In the invention of Patent Document 9, one of alignment members is limited to a material through which light is transmitted and the device itself is large-scaled, which are inconveniences.
As described above, in order to stably operate a micro-mirror device it is necessary to protect it from the influences of dust, external force and the like. In order to protect it from the influences of dust, external force and the like, roughly speaking, there are two of protection in the manufacturing process of MEMS structures and protection by packaging after the completion of the MEMS structure. However, traditionally, either of these two kinds of protection has some practical difficulty or problems as described above. Therefore, a method for easily achieving these two kinds of protection without any special material and any complicated and troublesome process is desired.
Patent Document 1: U.S. Pat. No. 4,229,732
Patent Document 2: U.S. Pat. No. 4,662,746
Patent Document 3: U.S. Pat. No. 5,817,569
Patent Document 4: U.S. Pat. No. 6,720,206
Patent Document 5: U.S. Pat. No. 6,753,037
Patent Document 6: U.S. Pat. No. 6,787,187
Patent Document 7: U.S. Pat. No. 7,071,025
Patent Document 8: U.S. Pat. No. 6,649,435
Patent Document 9: U.S. Pat. No. 6,947,200